In a discipline of sheet material irradiation, particularly with an electron beam, uniform beam and dose distribution across the entire surface is critical to achieve uniform product characteristics. Since the advent of charged particle and, especially, electron beam treatment of material, many devices and improvements thereon have been introduced to promote beam distribution control to achieve radiation dose uniformity. The first true advance involved controlled oscillatory scanning of the beam, exemplified in Robinson, U.S. Pat. No. 2,602,751. Although greatly enhancing the usefulness of scanning technology, problems with product uniformity still existed. Generally, these problems resulted from beam scanning geometries. In order to promote greater uniformity, adjunct devices were subsequently introduced. Such devices include reflected beam techniques such as that exemplified by Yehara, U.S. Pat. No. 3,942,017, deflecting scatter plates (see Robinson above), as well as a host of target product manipulation devices that move and twist the target relative to the scanning beam. Referring particularly to target product manipulation, the complex equipment employed is subject to mechanical breakdown and wear. Thus, serious maintenance problems arise, especially when breakdown occurs during processing. Not only must production be interrupted but also a significant quantity of target product may be lost.
Turning now to beam distribution control devices, although they enhance target product dosage uniformity, they often fail to achieve the objective of substantially ideal uniformity. Uniform dosage distribution is a function, both of the target material's dose tolerance and of the scanned beam characteristics. It is well known in the art that surface dose uniformity of an electron beam degrades as energies decrease. Hence, at beam energies under 1 MeV and, more particularly, at 300-400 KeV, a 5% or more variation of dosage uniformity is generally observable. This loss of uniformity results from the intensity loss of electrons upon passage through the electron beam's source window (generally formed from titanium foil and the like) compounded by the diminished scattered electrons impingement at the scan boundaries.
Referring first to the intensity loss, the apparent thickness of the beam source window and air space between the window and target progressively increases toward the boundaries of the scan angle. Although generally constructed to possess a minimum thickness, electron scattering is generated both by the window and by the depth of the atmosphere between the window and target product surface. Basically, the greater the apparent thickness, the greater the degree of electron scatter and the greater the loss of beam dosage intensity along the scan boundaries. This loss is easily expressed by a simple arithmetic proportionality: EQU intensity .apprxeq.l/cos .alpha.
where .alpha. equals the scan angle at a given point. Hence, as the beam is scanned across the entire product path, electron scatter increases from a minimum at a normal angle of incidence to a maximum at the sweep angle boundaries. Conventionally, the product edges correspond with the scan boundary. Thus, the increase in scattering results in an effective dosage loss and corresponding reduced product irradiation uniformity at the product edges.
The second major contribution to the non-uniform target exposure from the increasing apparent thickness, particularly in the case of flat or sheet-like material, is the loss of dosage intensity from scattered electrons. As identified above, as the beam approaches the product edge (scan boundary), the apparent thickness of the window and air space between the window and material increases. Scattering is particularly detectable with beams having energies under 1 MeV. A measurable portion of electrons scattered by the window and air gap during scanning, impinge on the product peripheral to the primary beam. However, at the edge of the target, such scattered electrons will impinge on air or a surface adjacent to the product. Thus, the edges of the target do not receive reinforced electron scatter from the beam as it moves progressively across the target and the contribution to actual dosage will be absent at the target edges.
This phenomenon has been recognized in the art and has been addressed by use of corrective adjunctive equipment, the most common being the use of electrified scatter plates and wedge magnets. In the case of electrified scatter plates, the primary electron beam impinges on a plate positioned below a scan horn window causing the generation of secondary electrons (see FIG. 2). A portion of the secondary electrons which are released from the scatter plate isotropically, impinge upon the product edges and provide a corresponding increase in product edge irradiation and, hence, product uniformity. Although generally acceptable, the technique suffers from the shortcoming of producing secondary electrons that scatter in all directions from the scatter plate. More importantly, secondary electrons generated by the scatter plate method do not possess the same energy as the primary electrons thereby resulting in a lesser degree of penetration of those electrons at the product edges. Therefore, ideal uniformity is not achieved.
The use of wedge magnets positioned peripherically within the scan horn and immediately above the window (see FIG. 3 herein), the second principal conventional corrective technique to provide increased beam uniformity, is clearly illustrated in U.S. Pat. No. 2,993,120 to Emanuelson. The magnets generate magnetic flux in the scan horn base corresponding with the height of the magnet. The wedge magnets technique is intended to produce a minimal transverse magnetic field at the center of the scan horn (minimal height) corresponding with a normalized electron beam and progressively increased intensity toward the scan periphery (maximum magnet height). This technique overcomes the problem of energy loss associated with the scatter plate apparatus but does not solve the electron scatter problem at the target edges as identified above. Hence, the target product edges are still deprived of an equivalent amount of irradiation as compared to the center portion of the target product. The foregoing techniques share the common problem of failing to achieve uniform product irradiation due to non-uniform beam distribution and non-uniform particle scatter across the entire scan.